Method of and apparatus for the production of cellulose pulp



cs. A. RICHTER 1,735,015 METHOD OF AND APPARATUS FOR THE PRODUCTION OF CELLULOSE PULP Nov. 12, 1929.

Filed Aug. 23, 1928 Patented Nov. 12, 1929 UNITED. STATES PATENT OFFICE GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOB TO BROWN COMPANY,

OF, BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE METHOD OF AND APPARATUS FOR THE PRODUCTION OF CELLULOSE PULP Application filed August 23, 1928. Serial No. 301,578.

This invention relates in its broadest as pects to a process of indirectly heating a pool of liquid by continuously removing liquid from the pool, passing it through an indirect heater, and returning it to the pool, and has for its object the realization of a rapid heating of the pool of liquid with a given heater when the rate at which liquid may be withdrawn from the pool is limited. In accordance with the present invention, this object is attained by recirculating liquid delivered from the heater through the same heater along with the liquid withdrawn from'the pool, such recirculation preferably being at a much higher rate than that at which liquid may be withdrawn from the pool, the liquid delivered from the heater which is not recirculated being returned to the pool. Such a process makes possible high heater efiiciency, since a given volume of liquid passes through the heater a number of times before being returned to the pool.

The process of the present invention may be applied to great advantage in the chemical pulp-making industry when the process known as indirect cooking is being practised. This process involves continuously pumping liquor from the bottom of the digester through a tubular heater outside of the digester, and returning the liquor to the top of the digester, a suitable screen being provided at the bottom of the digester to keep the chips and pulp in the digester so as to allow only liquor to be withdrawn by the pump. There are certain advantages in indirect cooking which are recognized by those uct.

skilled in the art. Briefly stated, this process permits cooking of the chips under substantially uniform conditions, since dilution of the liquor is avoided and substantially uniform temperature conditions are produced throughout the digester owing to a positive rather than a natural circulation of the liq; nor from the bottom to the top of the digester. Pulpmakers therefore employ the indirect cooking process when a uniformly cooked, reproducible pulp is desired as the final prod- In carrying out an indirect cook, the procedure is substantially as follows. The digester is charged with chips and liquor in suitable proportions, and the liquor passing through the screen at the bottom of the digester is continuously pumped through the.

external heater while steam under suitable pressure is being passed through the heater tubes, whereupon-the heated liquor is returned to the top of the digester. The circulation of liquor from the bottom of the digester through the heater to the top of the 'digester is continued until the digester contents have been brought to the temperature and pressure conditions desired for cooking, for instance, a temperature of about 335,F.

and a pressure of about pounds gage when kraft pulp is being produced. When these conditions have been produced in the digester,

liquor is then removed from the digester andment, when indirect cooking is being practised. The time necessary to reach the de sired temperature and pressure conditions in the digester de ends upon the rate of heat transfer in the eater through which the liq uor is being passed. The rate of heat transfor is affected by three factors, viz:

(1) The amount of heating surface in the heater,

(2) The difi'erence in temperature between the steam and the liquor passing through the heater,

(3) The velocity at which liquor is passing through the heater,

The rateof heat transfer may therefore be increased by:

(1) Employing a heater having a larger heating surface,

(2) Raising the temperature of the steam,

(3) Increasing the velocity of the liquor through the heater.

The use of a larger heater for the purpose of materially shortening the time necessary to reach the desired conditions involves cou siderable expense. Again, it is usually not feasible to employ steam at a PIGSSUIGWlllCh would result in a material shortening of this time. The velocity at which liquor is circuated through a heater of a given shape and size is limited by the maximum rate at which liquor can pass through the digester screen, and it would be impractical to use a long heater of comparatively small cross-sectional area, owing to the difficulty of cleaning such a heater and expansion trouble resulting from the use of long heating tubes.

Inaccordance with the present invention, the liquor delivered from the heater is continuousl pumped through the same heater along with the liquor normally being pumped therethrough from the bottom of the digester, but at a much higher velocity, so that a given volume of liquor passes through the heater a number of times before being returned to the top of the digester. Such a process makes possible greatly increasing the rate of heat transfer to the liquor and hence materially shortens the time necessary to come up to the desired temperature and pressure conditions in the digester.

A better understandingofv the present invention may be obtained by the following more complete description thereof when considered in conjunction with the accompanying drawing, which illustrates more or less conventionally the apparatus employed.

Referring to the drawings, 1 indicates a digester of the usual type, provided at its bottom with a conical screen 2. The liquor is strained free of chips and pulp as it passes through the screen into an outlet pipe 3, com municating with the bottom of the digester and with a pump 4 which continuously forces the liquor through a suitable indirect heater 5' outside of the digester, shown having a steam inlet 6 and outlet 7 and a liquor inlet 11 and outlet 8. Liquor delivered from the outlet 8 passes through a pipe 9 into a pump 10. which returns the liquor into the inlet 11 'along with liquor being removed from the bottom of the digester by the pump t, any

liquor not handled by the pump 10 being delivered to the top of the digester through a ipe 11, which may take a sinuous form, as illustrated, so as to allow for expansion. The

circulat ng capacity of the pump 10 is not limited bv the rate at which liquor is passing through the screen 2, and so may be designed to circulate the liquor throughthe heater at much higher velocity than that of the liquor handled by the pump 4. For instance, in actual practice, the pump4 maybe designed to handle about 600 gallons of liquor per minute, which rate is approximately the maximum obtainable when using a screen of the usual type at the bottom of a large commercial digester, while the pump 10 may be designed to circulate liquor at the rate of 2,000 gallons per minute, so that thetotal cirliquor per minute.

culation through-the heater is 2600 gallons of The present invention may thus eifect an approximate quadrupling of the velocity of liquor through the heater and an approximate doubling of the rate of heat transfer to the liquor. The volume of liquor being removed from the bottom of the digester, then heated, and delivered to the top of the digester is no greater when using the pump 10 tl1an when using only the pump 4, as the volume of liquor continuously being diverted into the heating-system, then heated, and returned to the digester is not influenced by operation of the pump 10, which'serves merely as a means of increasing the velocity of liquor through the heater.

After the digester contents have been brought to the desired temperature and pressure condition, operation of the pump 10 may be discontinued, but operation of the pump 4 may be continued, so as to maintain such temperature and pressure throughout the entire cook. The pump 10 may also be kept in operation throughout the entire cook if heat losses during cooking require more heat transfer to the cooking liquor than that ohtainable when using only the pump 4.

The advantage of the process of the present invention is that it makes possible a material decrease in the time necessary for pulping the raw material, as the coming-up-to-temperature and -pressure time may be reduced to one-half that ordinarily required, thus making possible higher production wit-h a given digester capacity; This advantage is of greater significance than may off-hand appear, for pulp digesters are usually built as large units, and a comparatively small increase in production effects a considerable saving over long periods of time.

Having thus described certain embodiments of this invention, it is evident to those skilled in the art that various changes and modifications might be made therein without departing from the spirit or scope of invention as set forth in the appended claims.

What I claim is:- 1. A- process of indirectly heating a pool of liquid from which liquid may be withdrawn at a limited rate, which comprises withdrawing liquid from said pool and passing it through an indirect heater at such limited rate, recirculating liquid delivered from such heater through the same heater, and returning liquid delivered from the heater to the-pool.

2. A process of indirectly heating a pool of liquid from which liquid may be withdrawn at alimited rate, which comprises withdrawing liquid from said pool and passing it through an indirect heater at such limited rat-e, recirculating liquid delivered from said heater through the same heater at a much higher rate along with the liquid being withdrawn from said pool, and returning liquid delivered from the heater to the pool.

3. A process of indirectly cooking a charge of cellulosic material and liquor, which comprises removing liquor from said charge, passing the liquor through an indirect heater, recirculating liquor delivered from the heater through the same heater, and returning liquor delivered from the heater to the charge.

4. A process of indirectly cooking a charge of cellulosic material and liquor, which comprises continuously screening liquor from said charge, continuously passing the liquor through an indirect heater substantially at the rate at which it is being screened, continuously recirculating liquor delivered from the heater through the same heater but at a much higher ratealong with the liquor being screened, and continuously returning liquor delivered from the heater to the charge.

5. Apparatus of the class described, comprising in combination, a pulp digester, means for removing liquor from said digester, an indirect heater through which said liquor mav be passed, means for recirculating liquor delivered from said heater through said heater, and means for passing liquor delivered from said -heater to said digester.

6. Apparatus of the class described, comprising in combination, a pulp digester, a screen at the bottom of said digester, a ump for continuously removing screened liquor from said digester, an indirect heater through which said-liquor may be passed from said pump, a pump for recirculating liquor-do livered from said heater through said heater at higher velocity than from said first-mentioned pump, and means for passing liquor delivered from said heater to the top of said digester.

In testimony whereof I have aflixed my signature.

GEORGE A. RICHTER. 

